In the past, furnaces using preheated combustion air had two regenerative means wherein the air is predetermined in one means, while the flue gases heat the other means. Then the flue gas was vented to the atmosphere. The mass flow rate of the hot gases in the furnace was limited by the end temperature of the combustion chamber. If a hotter chamber was required, one would obviously increase the air-fuel rate which in turn increased the mass flow rate within the chamber. Conversely, if one desired to increase the mass flow rate to increase heat transfer to the terms to be heated, one would also in these prior art systems would increase the temperature. An increase in temperature many times can not be tolerated by the products to be heated.
In addition, if one was heat-treating, or melting highly active metal, such as aluminum, special fluxes would be required in prior art systems to prevent oxidation of the metal. If one would use a fuel rich combustion mixture to prevent oxidation of the metal, energy would be wasted in these prior art systems.